Fluid couplers in the nature of torque converters are widely employed in vehicles to transfer torque between the engine and the transmission. In additional to this fundamental purpose, torque converters serve two other primary functions. First, they provide a means by which to effect a smooth coupling between the engine and the transmission. Second, they provide hydraulic torque multiplication when additional performance is desired.
The typical torque converter being utilized in vehicular drive trains normally has three major components--vis.: a centrifugal pump or torque input member (commonly designated as the impeller); the driven or torque output member (commonly designated as the turbine); and, a reaction member (commonly designated as the stator) that is interposed between the impeller and the turbine to effect a more favorable direction for the flow of hydraulic fluid exiting the turbine and returning to the impeller.
The vanes of the impeller are generally secured to the cover of the torque converter. Inclusion of the cover as a component of the impeller assembly has simplified sealing the torque converter so that the impeller assembly may be contained within a housing that is filled with hydraulic fluid. Although hydraulic fluid is supplied to the torque converter under pressure by a pump and torque converter valve arrangement, dynamic circulation of the hydraulic fluid through the torque converter is effected by rotation of the cover to which the impeller vanes are affixed, and the dynamically circulating hydraulic fluid impinges against the opposed vanes of the turbine to effect rotation of the turbine in response to rotation of the impeller. The torque converter is thus a closed system with the impeller operatively connected to a source of input torque and with the turbine member operatively connected to deliver the output torque, as required.
Typically, the cover and the vane elements of the impeller that are attached thereto--the combination of which constitutes the impeller assembly --are affixed to a flex plate that is bolted to the crankshaft of the engine. The turbine, on the other hand, is connected to an output shaft which exits the torque converter to serve as the input shaft of the vehicle transmission.
Because of the aforesaid fixed mechanical connection between the engine and the impeller, the impeller will rotate at engine speed whenever the engine is operating. It is this rotation of the impeller which causes it to operate as a pump, and particularly a centrifugal pump. That is, the impeller ingests pressurized hydraulic fluid present at the central or hub portion thereof and discharges hydraulic fluid axially into the turbine at the radially outer rim of the impeller assembly. Although the axial component of the hydraulic fluid discharging from the impeller moves the hydraulic fluid axially into the turbine, rotation of the impeller also imparts a circumferential or centrifugal component to the hydraulic fluid as it exits the impeller. As the hydraulic fluid exiting the impeller engages the turbine, the kinetic energy of the moving hydraulic fluid urges the turbine to rotate in response to rotation of the impeller.
When the vehicle is not moving, and even though the engine is idling, the impeller is not spinning at a sufficient angular velocity to supply the energy necessary to overcome the static inertia of the vehicle. In that situation, therefore, the hydraulic fluid simply flows through the turbine, and ideally the turbine does not rotate. This allows the vehicle to remain at rest, even though the transmission has been shifted into a selected drive range and the engine is running.
As the throttle is opened, however, the rotational speed of the engine, and therefore the impeller, increases. At some rotational speed of the engine, sufficient energy is being imparted to the turbine so that it will be able to overcome the static inertia that theretofore prevented the vehicle from moving. At that time, the energy transferred from the impeller to the turbine will be delivered to the drive wheels through the transmission.
Kinetic energy is most effectively imparted to the turbine when the hydraulic fluid circulating within the torque converter follows the contours of the turbine blades and the shell from which they are presented, and then leaves the turbine. The most effective configuration for the contoured surfaces of the impeller vanes have evolved over years of testing and experience. Perhaps, the most effective vanes are the bulbous-nosed curvilinear vanes having a hydrofoil configuration, but the cost to fabricate this efficient contour is comparatively high. In any event, the configuration of the turbine causes the fluid passing therethrough to exit in a direction that is generally inappropriate to that direction at which one would prefer to have the hydraulic fluid re-enter the impeller. Accordingly, were the fluid to re-enter the impeller in that direction, the fluid would strike the vanes of the impeller in a direction that would be detrimental to the rotation of the impeller.
In order to minimize the problems resulting from the undesirable direction at which the fluid would enter the impeller as it exits the turbine without any redirection, a stator is generally interposed within the path which the hydraulic fluid must traverse between its exit from the turbine and its re-entry into the impeller. In fact, the stator redirects the hydraulic fluid which has exited the turbine so that the fluid will enter the input of the impeller in a direction that will cause the fluid to assist the engine in turning the impeller. The force which the hydraulic fluid thus imparts to the impeller comprises one source for additional kinetic energy being applied to the turbine. It is this additional energy applied to the impeller which results in an increase in the force applied to drive the turbine--thereby accomplishing torque multiplication.
In order to obviate the costs of casting, machining and finishing vanes having sophisticated contours, noncontoured blades are being substituted for the contoured vanes, and the blades are, for the most part, being stamped from sheet metal and then secured to the shell of the impeller. However, as might be expected, such sheet metal blades provide lower efficiency than the more expensive contoured vanes. This result is particularly apparent during those conditions of operation at which the pressure of the hydraulic fluid within the torque converter falls within the lower portion of the acceptable operating range.
In order to achieve utmost economy, torque converters are often provided with a clutch arrangement that effectively locks the impeller and the turbine into a unitary rotating mass when "slip" between the impeller and the turbine is no longer required for smooth coupling. Typically, the torque converter clutch is activated to effect unitary rotation in response to reduced hydraulic pressure within the torque converter. Thus, should the lowered pressure be the result of the reduced efficiency resulting from the use of a noncontoured impeller and/or turbine blades rather than the result of the impeller and the turbine operating at near unitary rotational speeds, the torque converter clutch could inadvertently activate before such activation was desired.
It must be appreciated that torque converters are not closed systems. In practice, torque converters require a source of clean pressurized hydraulic fluid. A pump draws hydraulic fluid from the transmission pan or from a sump, and delivers hydraulic fluid at a predetermined pressure. A torque converter control valve has been traditionally employed to maintain the preselected pressure of the hydraulic fluid supplied by the pump. The pressurized hydraulic fluid is supplied to the converter, where the hydraulic fluid is used to effect a hydraulic torque transfer between the impeller and the turbine within the torque converter. Thereafter, the fluid is directed through a cooling system to the pump inlet and then recycled. The pump which delivers the pressurized hydraulic fluid is normally driven by the engine of the vehicle. As such, pressures at the outlet of the pump will normally vary, at least to some degree, in response to engine speed.
Torque converter control valves are usually spring balanced and are opened to permit flow to the torque converter only after the pressure at the outlet of the pump reaches some predetermined value. Thus, it is only after the pressure of the hydraulic fluid at the outlet of the pump attains the desired magnitude that the torque converter control valve will open to permit the flow of pressurized fluid to the torque converter. As the pressure continues to increase, the torque converter valve directs excess fluid to the inlet side of the pump.
Such arrangements have worked quite well in the past, and continue to work well for torque converters having sophisticated impeller vanes. However, when one substitutes noncontoured blades for the contoured vanes of an impeller, it has been found that the historic torque converter control valves are unable to maintain the hydraulic fluid within the torque converter at the pressures necessary to accommodate the efficiency differential between impeller vanes and impeller blades.
In addition, those torque converters which incorporate clutches that interact between the impeller and the turbine generally require that some minimum pressure be maintained within the torque converter.